Photovoltaic devices such as solar cells (and modules thereof) are known in the art. Example solar cells are disclosed in U.S. Pat. Nos. 4,510,344, 4,806,436, 6,506,622, 5,977,477, and JP 07-122764, the disclosures of which are all hereby incorporated herein by reference in their entireties.
Substrate(s) in a solar cell/module are often made of glass. Incoming radiation passes through the incident glass substrate of the solar cell before reaching the active layer(s) (i.e., photoelectric transfer film such as a semiconductor) of the solar cell. In particular, the power output of a solar cell or photovoltaic (PV) module may be dependant upon the amount of light, or number of photons, within a specific range of the solar spectrum that pass through the incident glass substrate and reach the photovoltaic semiconductor.
In order to enhance solar cell or PV absorption of light, mirrors or lenses may be applied in a solar cell system, e.g., in connection with a concentrated solar power (CSP) system that helps to concentrate a large area of sunlight onto a small area. Concentrated light may be converted to heat, which may drive a heat engine (such as, for example, a steam turbine) that may be operatively connected to an electrical power generator to produce electrical power.
High quality mirrors are highly desirable in order to achieve more efficiency in energy conversion. However, mirror reflections and energy conversion efficiencies are often reduced by dust and other alien particles that adhere to the mirror surface. The particles stack on the surface over time, and create a barrier between light and the active layers under the surface.
It is known to address the contamination issue by using mirrors that may exhibit hydrophobic properties. A hydrophobic mirror surface may exhibit a higher resistance to the attack from dust particles by having a repelling force between mirror surface and dust particles. Hydrophobic properties may also allow less condensation of water and fog, and reduce the likelihood of snow or other frozen material collecting on the mirror surface. Both functions may improve mirror reflection capabilities, and improve power generation efficiencies.
Furthermore, hydrophobic properties of the surface may create a self-cleaning capability. The hydrophobic property may facilitate a rolling action of water droplets on the surface, instead of a sliding action. The rolling of water droplets may effectively remove the dust and unwanted particles on the surface.
Surface energy, surface roughness, and homogeneity are surface properties that can be adjusted and have an impact on hydrophobicity. For example, a surface with a high degree of roughness and low surface energy may show super-hydrophobicity. The highest reported contact angle for a sessile drop of water on a smooth surface is about 120°. However, water contact angles as high as 170° has been achieved with rough and low surface energy materials. Surface roughness produced by a fractal structure may be a factor in the increase of contact angle for a sessile drop of water.
Hydrophobic surfaces have been developed through numerous methods, including plasma etching, plasma deposition, laser treatment, sol-gel processing, anodic oxidation of aluminum, chemical etching and chemical grafting. Among them, the sol-gel approach demonstrates several advantages over other methods, such as, for example, (1) being a simpler process; (2) having a lower cost; (3) being more applicable to implement in a large scale production process; (4) processing at low temperature; and (5) making it easy to combine different materials.
Although hydrophobic coatings are known, further improvements are still desirable. For instance, it would be desirable to provide high water contact angle coatings that have indexes of refraction compatible with the mirrors used in solar applications (including concentrating solar power applications) and that help maintain high reflectivity by keeping the mirrors cleaner than they otherwise would be without such coatings by virtue of the anti-soiling functions. In this regard, in certain example embodiments, a method to enhance the hydrophobic properties of a coated substrate surface is provided. More particularly, the enhancement of anti-soiling and self-cleaning capabilities of surfaces becomes possible through the selection of alkylsiloxane mixtures that are aged and/or cured at different times. In addition to providing anti-soiling capabilities, the coatings of certain example embodiments advantageously achieve good average total reflection and refractive index values. In addition, in certain example embodiments, the water contact angle of sessile drops of water on the coated article surface corresponds to a hydrophobic coating that is in some ways comparable to surface properties of a lotus leaf that allows water droplets (such as, for example, rain drops, etc.) to roll off its surface.
One aspect of certain example embodiments relates to methods of providing an alkylsiloxane sol mixture and/or coating procedures that make a coating that exhibits hydrophobic surface properties on a major surface of a substrate.
Another aspect of certain example embodiments relates to a sol composition that includes a mixture of at least two alkylsiloxane chemicals aged and cured for short periods of time, to obtain a hydrophobic property.
Certain example embodiments of this invention relate to a method of making a coated article comprising a glass substrate supporting a coating. A sol is wet-applied, directly or indirectly, on a major surface of the substrate, the sol comprising at least first and second alkylsiloxane chemicals, with the first and second alkylsiloxane chemicals having tetra-alkoxysiloxane and tri-alkoxysiloxane structures, respectively. The sol is dried and/or cured to form the coating. The sol is aged for no more than five months prior to the wet-applying.
According to certain example embodiments, the alkylsiloxane chemicals are provided at substantially equal weight percentages.
According to certain example embodiments, alkylsiloxane chemicals are selected from the group consisting of octyltrimethoxysiloxane (OTMOS), pentyltriethoxysiloxane (PTEOS), 3,3,3-trifluoropropyl trimethoxysiloxane (TFTMOS), tetraethyl orthosilicate (TEOS), and combinations thereof.
Certain example embodiments of this invention relate to a method of making a coated article comprising a glass substrate supporting a coating. A sol is wet-applied, directly or indirectly, on a major surface of the substrate, with the sol comprising TEOS and OTMOS. The sol is dried and/or cured to form the coating. The coating has an initial contact angle of 100-131 degrees.
Certain example embodiments of this invention relate to a method of making a mirror. A thin film coating is disposed on a first major surface of the substrate, with the thin film coating having a reflectivity of at least about 85%. A sol is wet-applied, directly or indirectly, onto the thin film coating. The sol comprises at least first and second alkylsiloxane chemicals, with the first and second alkylsiloxane chemicals having tetra-alkoxysiloxane and tri-alkoxysiloxane structures, respectively, and with the sol having been aged for no more than three months prior to the wet-applying. The sol is dried and/or cured to form an anti-soiling coating that at least initially has a contact angle of greater than 100 degrees.
According to certain example embodiments, reflection from the mirror is no more than 0.15% lower than the reflection would be if no anti-soiling coating were present.
In certain example embodiments, a sol composition is provided. At least two alkylsiloxane chemicals are provided at substantially the same weight percents. A first alkylsiloxane chemical has a tetra-alkoxysiloxane structure, and a second alkylsiloxane chemical has a tri-alkoxysiloxane structure. The sol composition is aged less than 3 months and has a cure time less than 10 minutes.
In certain example embodiments, a coated article is provided. A multi-layer thin film coating is disposed, directly or indirectly, on a first major surface of a substrate, with the thin film coating having a reflectivity of at least about 85%. A wet-applied anti-soiling coating is formed from a sol aged for no more than about three months prior to the wet application and comprising tetra-alkoxysiloxane and tri-alkoxysiloxane components at least initially provided in substantially equal weight percents. The anti-soiling coating has an initial contact angle theta greater than 100 degrees, a refractive index less than 1.3, a thickness of 60-100 nm, and a root mean square roughness of 3-6.5 nm.
Certain example embodiments may relate to including a substantially similar weight percentage of the alkylsiloxane chemicals in the sol. In addition, or in the alternative, certain example embodiments may have a curing time of preferably less than 25 minutes, more preferably less than 15 minutes, and still more preferably less than 10 minutes. In addition, or in the alternative, certain example embodiments may include an aging time of preferably no more than about five months, more preferably no more than about 3 months, still more preferably no more than or equal to about 1 month, prior to the wet-applying.
It will be appreciated that the example aspects, embodiments, features, etc., may be combined in any suitable combination or sub-combination to provide yet further example embodiments.